SIGNIFICANCE: The World Health Organization (WHO) estimates that 600 million people world wide are exposed to pesticides on a regular basis, much of this exposure is to organophosphate pesticides (OP). Past studies have documented persistent neurological effects in previously OP poisoned workers and some with chronic OP exposure. The goal of this clinical epidemiological study is to determine whether farm workers chronically exposed to OP pesticides in field work have quantifiable nervous system abnormalities after an exposure season and whether these changes are persistent after a several month period of non-exposure. DESIGN: This cross-sectional study with a follow-up component will compare 50 female orchard farmworkers with chronic low-level OP exposure to age, education, and language matched unexposed referents. Exposure estimates' will be based on orchard spray logs from participating orchards, work questionnaires of subjects, and biological monitoring (cholinesterase testing). A neurological battery including the WHO Neurobehavioral Core Test Battery, vibratory threshold, color discrimination testing, nerve conduction and repetitive stimulation studies, and pattern reversal visual evoked potentials will be applied to both groups, both at the end of an exposure season and again following a 5-6 month period of non-exposure. Several biological markers which may serve as markers of susceptibility, physiological-effect, or dose will also be evaluated: cholinesterase, paraoxonase, chlorpyrifos oxonase levels, and muscarinic receptor density in lymphocytes. SPECIFIC AIMS: 1. Are there abnormalities in neurological (neurobehavioral, neurosensory, and neurophysiological) function in female farmworkers with chronic low-level 0P exposure immediately following an exposure season as compared to a reference group with no current and little or no historical 0P exposure? 2. Do identified abnormalities show a relationship with estimates of recent (past season) and historical exposure? 3. Do identified abnormalities persist after six months of non-exposure and do they show a relationship with estimates of historical exposure? 4. Does paraoxonase or chlorpyrifos oxonase activity level show any protective effect and thus help predict performance on a neurological battery when contrasted with historical exposure? 5. Is muscarinic receptor density affected by chronic exposure to 0P pesticides as estimated by work time in fumigated orchards?